1. Field of the Invention
This invention relates to a sheet feeding apparatus that separates and supplies paper from a bundle of stacked sheets of paper one sheet at a time. For example, the invention applies to an apparatus that supplies either documents stacked in a document tray or sheet papers stacked in a paper feed unit in an electrostatic copying machine.
2. Description of Related Art
A sheet feeding apparatus for documents in an electrostatic copying machine is described below by example.
As a sheet feeding apparatus that feeds document sheets stacked in a document tray from the lowermost portion in order one at a time in an electrostatic copying machine there is an apparatus that injects air to the front edge (downstream side of feeding direction) of the document bundle to prevent duplicate feeds of the documents. In this case, it is necessary to set the injection quantity of air to an appropriate amount and, a construction to achieve this is shown in Japanese Unexamined Patent Publications No. 57-160837 and 60-36248.
If the construction mentioned in Japanese Unexamined Patent Publication No. 57-160837 is summarized, it is a construction that measures the frictional force between the stack (document bundle) and a tray and, depending on this frictional force, controls the injection quantity of air. The two methods below are mentioned as means to measure the frictional force for this case. One of the methods uses a solenoid to obtain time as frictional force from after the stack is ejected at a fixed distance until it returns. If the injection quantity of air is correct and the stack floats from the tray in an appropriate state (if an adequate gap is formed between both), the stack returns in the correct time although, if the injection quantity of air is small, the stack will not return to its original position. The injection quantity of air is thus obtained. The other method uses a light sensor to directly detect the gap between the tray and the stack and to use this as a value corresponding to the frictional force.
Further, if the construction mentioned in Japanese Unexamined Patent Publication No. 60-36248 is summarized, it is a construction that measures the document stack stacked in a loading unit by a sensor as height, namely measuring the stack as height in response to the number of documents stacked to control the injection quantity of air in response to the height of this stack. Therefore, in this construction, the injection quantity of air is determined in response to the number of sheets comprising the document stack height to be loaded and, the loaded stack is made afloat by the air.
However, these conventional types of constructions have the following problems.
For the construction mentioned in Japanese Unexamined Patent Publication No. 57-160837, the stack must be floated completely to form the gap, thus making control difficult depending on the paper size or document quantity. For example, if the document size is small, the entire stack can float although, if the document size becomes large, floating the entire stack becomes difficult. In other words, if the injection quantity of air is raised even slightly too much with large sized documents, the documents will warp and then will not completely float. Further, if the stack is large (large quantity of document sheets), the stack can be floated by increasing the injection quantity of air. However, a great deal of time is required until the stack completely floats and the construction is not suitable for recent copying machines which must compete for the time until the first copy.
Moreover, for the construction mentioned in Japanese Unexamined Patent Publication No. 60-36248, the height of the stack is detected although, in an actual document, there are many occurrences of curls and other problems thus, errors occurred in detecting the size of the stack, which made it impossible to control a correct injection quantity of air. For example, if an upward curl occurred in the document, the height of the stack would be detected larger than the actual stack and the injection quantity of air would be set larger than necessary, resulting in the excessive document floating. This led to paper jams due to poor paper feed (paper supply mistake). Also, if an downward curl occurred in the document, the height of the stack would be detected smaller than the actual stack and the injection quantity of air would be insufficient, thus resulting in duplicate feeds.